I have tried to put the DECODE etc into the GROUP BY statement but I get an error - " not a valid GROUP BY clause"

Secondly, is there a way of modifying value in field A based on the value in field B

Erroneous example follows - in an effort to explain what I want to do (from above):

DECODE(PRIM_REASN,'',(DECODE(EVENT_DUR,<200,"Low",>200,"High"),PRIM_REASN)ie:If PRIM_REASN is null I want EVENT_DUR to be tested to determine the new value for PRIM_REASN.Except that tthe above won't work and I don't know what the right function/argument should be..

This is my first foray into SQL server, I am trying to migrate some access queries to work with web services and hence Xcelsius.